1. Field of the Invention
This invention relates to telephone ringing extenders and, more particularly, to ringing and protector interfaces between telephone loops and internal telephone subscriber wiring.
2. Description of the Prior Art
Registration requirements of the Federal Communications Commission require a two-wire interface between customer-provided equipment and the telephone network. On many telephone subscriber loops, particularly long loops, it has been common to connect the ringer in the subscriber telephone set between one side of the loop and ground potential. This arrangement increases the amount of current that can be delivered over the long loop to ring subscriber telephone sets which may include as many as five extension telephones. Such loops are found in rural and suburban areas and in central office installations utilizing fine gauge wire for intermediate length loops.
The ability of telephone subscribers to purchase their own telephone sets from any supplier further requires that telephone sets be of standard design and prewired in a standard configuration. As noted above, the two-wire interface implies that the telephone ringers are bridged between the tip and ring conductors, and that grounded ringers will not be used. Since it is expected that the wider and more dispersed availability of telephone sets will increase the average number of extension telephones at a subscriber location, it appears likely that this problem will be aggravated in the future. A current requirement is the ability to operate five ringers on a 2800 ohm subscriber loop.
One solution to the multiple ringer problem on long loops is disclosed in C. E. Jones, Jr. et al. U.S. Pat. No. 4,021,617, granted May 5, 1977. In the Jones, Jr. et al. arrangement, ringing signals are detected at the end of long subscriber loops by electronic detection circuitry. In response to such ringing signals, the ringer, connected to one of the loop conductors, is grounded to provide a low impedance connection for ringing return current. The Jones, Jr. et al arrangement, however, requires a three- or four-wire interface to the telephone set to provide the ground connection.